Is your dog due a season/heat at all?
Does her urine look normal?
Is she licking her vulva or paying lots of attention to it?
Thanks for the extra info. Itis possible that your dog does have cystitis, or the problem could be a vaginitis (an inflammation and infection of the inner wall of the vagina). It is also possible that she has a dermatitis or infection and inflammation of the skin of the vulva itself, making it sore and inflamed.
If you find that she starts to lick at it a lot, it would be better to try to stop this, as she can make the infection or inflammation worse. You could do this by adapting a pair of pants for her (cut a hole for the tail). It may also help to bathe the area with some cool water to reduce the heat and swelling.
If you can manage to, try to take a urine sample with you to the vet tomorrow. This will help the vet to establish whether the cause is urinary based, and will speed up the diagnosis if it is there to test at the time.